1. Field of the Invention
The present invention relates to a negative temperature coefficient thermistor consisting of LaCoO.sub.3 rare earth transition element oxide, capable of inhibiting a rush current.
2. Description of the Related Art
The LaCoO.sub.3 rare earth transition element oxide has a larger B constant than that of a conventional manganese spinel negative temperature coefficient thermistor material, and is capable of further reducing a resistance value of a thermistor element at a high temperature. Therefore, when an electric current is applied, it is possible to inhibit a self heat generation of a negative temperature coefficient thermistor element, thereby increasing a rated current value. For this reason, LaCoO.sub.3 rare earth transition element oxide is suitable for use as a material in forming a negative temperature coefficient thermistor element capable of inhibiting a rush current.
However, when external electrodes are to be formed on a negative temperature coefficient thermistor element consisting of a LaCoO.sub.3 rare earth transition element oxide, if Ag or Ag--Pd paste for forming a sort of thick film electrode is used which contains a kind of glass frit consisting of usual SiO.sub.2, PbO, Bi.sub.2 O.sub.3, an interface between a negative temperature coefficient thermistor element and the external electrodes will become non-ohmic, hence making the negative temperature coefficient thermistor element to have only an unstable resistance value. For this reason, a negative temperature coefficient thermistor element consisting of LaCoO.sub.3 rare earth transition element oxide is formed on the outer surfaces thereof with external electrodes which are obtained by using a thick film electrode formation paste not containing the above glass frit.
However, since the above negative temperature coefficient thermistor containing as its main component the above LaCoO.sub.3 rare earth transition element oxide, is provided with external electrodes which are formed by a fritless paste, there is only a lower adhesion strength between the negative temperature coefficient thermistor element and the external electrodes than that of a thick film electrode containing a common glass frit. When one tries to increase the adhesion strength between the negative temperature coefficient thermistor element and the external electrodes, it will not be sufficient if a sintering treatment is carried out at a temperature of 600 to 850.degree. C. for one hour (just like a process in which a common thick film electrode is formed). Instead, such a sintering treatment is needed to be conducted at a temperature of 900 to 1000.degree. C. for five hours. Hence, as a result, since a relatively long time is required in forming the external electrodes, there had been a problem that the external electrodes have to be formed with a high cost.
Moreover, there have been existing another problem which may be concluded as follows. Namely, if a negative temperature coefficient thermistor having formed thereon the external electrodes is attached to a circuit board, or if a lead wire is soldered on to a negative temperature coefficient thermistor having formed thereon the external electrodes, it is allowed to obtain a certain negative temperature coefficient thermistor product. If such a negative temperature coefficient thermistor product is continuously used under a temperature of 100.degree. C. or higher, a solder component such as Sn will diffuse on to the external electrodes, so that an element Ag forming the external electrodes will be corroded due to the solder. As a result, the external electrodes will have only a low strength, and the resistance value of the negative temperature coefficient thermistor will become high.